1. Field of the Invention
The present invention relates to a thin-film structure in which an insulating layer is formed on an electrically conductive layer, a bump formed on the surface of the electrically conductive layer is exposed at the surface of the insulating layer, and the surfaces of the bump and the insulating layer are flush with each other so as to form a flat face, and more particularly, relates to a thin-film structure in which an oxide layer formed on the bump exposed at the surface of the insulating layer can be reliably removed.
2. Description of the Related Art
FIG. 9 is a cross-sectional view of a conventional thin-film structure. In this thin-film structure, for example, an insulating layer 3 composed of Al2O3 or SiO2 is formed on an electrically conductive layer 2, such as an elevating layer, composed of copper or the like and in contact with a lead layer extending from a coil layer of an inductive magnetic head, and a bump 1 is formed on the surface of the electrically conductive layer 2. The bump 1 is exposed at the surface of the insulating layer 3, and the surfaces of the bump 1 and the insulating layer 3 are flush with each other so as to form a flat face.
The bump 1 constructing a conventional thin-film structure is a single-layer structure formed by an isotropic plating of copper-or--an electrically conductive material containing copper. An electrode layer 4 electrically contacting the bump 1 is formed on a flat face 1a of the bump 1. The electrode layer 4 is made of gold.
A thin-film device, such as an inductive magnetic head, is connected to a wiring member (not shown) which transmits signals at the electrode layer 4 of the thin-film structure. Recording signals inputted via the electrode layer 4 are transmitted through the bump 1 and the electrically conductive layer 2.
The thin-film structure can be used in the construction of an inductive head of a magnetoresistive (MR)/inductive hybrid head shown in FIGS. 10 and 11.
FIG. 10 is a plan view showing a so-called MR/inductive hybrid head in which a recording inductive head H2 is disposed on a reading MR thin-film magnetic head H1. FIG. 11 is a cross-sectional view of the MR/inductive hybrid head taken along the line XIxe2x80x94XI in FIG. 10.
As shown in FIG. 11, the recording inductive head H2 of the MR/inductive hybrid head is composed of a lower core layer 5, a gap layer 6, a coil layer 7, an insulating layer 8, an upper core layer 9, a lead layer 10, and an insulating layer 3 composed of Al2O3 in a laminated structure.
The coil layer 7 induces a recording magnetic field to the lower core layer 5 and the upper core layer 9. In FIG. 10, for ease of illustration in the figure, the coil layer 7 is drawn as concentric circles instead of as a coil.
The coil layer 7 is in electrical contact with the lead layer 10 at an central edge 7a of the coil layer 7, and the lead layer 10 is connected to a bump 1 via the electrically conductive layer 2 at the other edge of the lead layer 10 opposite to the edge thereof contacted with the central edge 7a of the coil layer 7. The electrically conductive layer 2, which is an elevating layer, is simultaneously formed, when the coil layer 7 is formed, by plating using the same material as is used for the coil layer 7.
In the conventional thin-film structure shown in FIG. 9, the bump 1 formed on the electrically conductive layer 2 is exposed at the surface of the insulating layer 3, and the electrode layer 4 in electrical contact with the bump 1 is used as an external electrode.
FIGS. 12 to 14 are cross-sectional views-of the thin-film structures in FIG. 9 in a manufacturing method therefor.
First, a resist layer 50 for forming the bump is formed on the electrically conductive layer 2, and an opening is formed in the resist layer 50 at an area at which the bump is to be formed. In the opening, the single-layer bump 1, as shown in FIG. 12, composed of copper, or composed of an electrically conductive material containing copper, is formed by isotropic plating. The height h1 of the bump 1 is, for example, 40 xcexcm. After forming the bump 1, the resist layer 50 for forming the bump is removed.
After forming the bump 1, as shown in FIG. 13, the insulating layer 3 composed of Al2O3 or SiO2 is formed on the electrically conductive layer 2 and the bump 1.
Next, the insulating layer 3 is polished so that the surface of the bump 1 is exposed, for example, to the level indicated by the line Axe2x80x94A in FIG. 13. The exposed face of the bump 1 is flush with the surface 3a of the insulating layer 3 so as to form the flat face 1a, as shown in FIG. 14.
Finally, the electrode layer 4 is formed on the flat face 1a of the bump 1, whereby the thin-film structure shown in FIG. 9 is completed.
However, in the step shown in FIG. 14, the flat surface 1a of the bump 1 is exposed to the air, and an oxide layer may form thereon. In particular, when heat is applied in a rinsing/drying process or the like after a polishing process for planarizing the surface 3a of the insulating layer 3, the flat face 1a of the bump 1 is susceptible to forced oxidation.
When the flat face 1a of the bump 1 is oxidized, an oxide layer forms on the flat face 1a. In the case in which the electrode layer 4 is formed on the surface of the flat face 1a with the oxide layer thereon, the cohesion and the electrical conduction between the bump 1 and the electrode layer 4 are degraded, so that electrical contact failure readily occurs, and direct current resistance of the thin-film device becomes unstable, and as a result, the recording/reading characteristics are degraded.
When the oxide layer forms on the flat face 1a of the bump 1, there are methods for removing the oxide layer by using ion-milling, sputter etching, and the like. However, in the case in which the bump 1 is the single layer structure composed of copper or an electrically conductive material containing copper, the thickness of the oxide layer formed by exposure to the air varies in accordance with the conditions when the oxide layer was formed, and as a result, the thickness of the oxide layer on the flat face 1a of the bump 1 cannot be predicted.
Consequently, when the conditions for ion-milling are determined so as to remove a predetermined thickness of the oxide layer formed on the flat face 1a of the bump 1, the oxide layer may not be reliably removed since a predetermined thickness to be removed is too small, or conversely, an area of the bump 1, which is not oxidized, may be removed. Accordingly, there is a problem in that the characteristics of the thin-film structures vary from product to product.
In order to solve the conventional problems described above, it is an object of the present invention to provide a thin-film structure and a manufacturing method therefor, in which an oxide layer formed on a bump constructing the thin-film structure of the thin-film device can be reliably removed, and cohesion and electrical conduction between the bump and an electrode layer are improved, whereby electrical contact defects therebetween can be reduced.
A thin-film structure of the present invention comprises an electrically conductive layer, an insulating layer formed on the electrically conductive layer, and the bump formed on the surface of the electrically conductive layer, in which the bump is exposed at the surface of the insulating layer and the surface of the bump is flush therewith so as to form a flat face. The bump comprises an electrically conductive material layer and a protective layer, in which the protective layer covers the surface the electrically conductive material layer, is composed of a material which is not as easily oxidized as the electrically conductive material layer, and is on the flat face.
In a process for forming the thin-film structure of the present invention, after the bump composed of Al2O3, SiO2, or the like is covered with the insulating layer, the insulating layer and the bump are polished, so that the bump is exposed at the surface of the insulating layer. The exposed face of the bump is flush with the surface of the insulating layer of the thin-film device so as to form the flat face.
The bump of the present invention is composed of the electrically conductive material layer and the protective layer which is composed of a material not as easily oxidized as the electrically conductive material layer and which is on the flat face.
Consequently, even though the flat face of the bump is exposed to the air at room temperature or to heated air, formation of the oxide layer on the flat face of the bump can be suppressed compared to that in a conventional one.
When the insulating layer and the bump are polished so that the bump is exposed at the surface of the insulating layer and the surface of the bump is flush with that of the insulating layer, the electrically conductive material layer may be exposed at the flat face of the bump when the thickness of the protective layer is too thin.
Accordingly, the protective layer preferably has a minimum predetermined thickness. In the case in which the thin-film structure of the present invention is used in the construction of a thin-film device, such as a thin-film magnetic head and a thin-film transistor, the thickness of the protective layer in an area other than the flat face of the bump is preferably, for example, not less than 5 xcexcm.
In addition, the electrically conductive material layer constructing the bump preferably comprises a copper layer or a copper-containing layer, and the protective layer preferably comprises a nickel layer or a nickel-containing layer.
When the electrically conductive material layer of the bump is formed using copper which has superior electrical conductivity due to the low resistance thereof, electrical conductivity of the entire thin-film structure is superior and high-frequency characteristics of the thin-film device are also improved.
Even though the nickel layer or the nickel-containing layer is exposed to the air at room temperature or to heated air so that the oxide layer forms thereon, it is believed that the oxide layer will not exceed a certain thickness.
For example, in the case in which the protective layer is composed of nickel or an electrically conductive material containing nickel, it has been experimentally confirmed that the thickness of the oxide layer formed on the surface of the flat face of the bump is at most approximately 3.0 xcexcm.
Consequently, when approximately 3.0 xcexcm of the oxide layer formed on the flat face of the bump is removed from the surface thereof using a dry etching method, such as ion-milling or sputter etching, the oxide layer is reliably removed from the surface of the flat surface of the bump.
Furthermore, it is more preferable that the electrically conductive material layer constructing the bump be the copper layer or the copper-containing layer, and that the protective layer be a gold layer or a gold-containing layer formed on the nickel layer or the nickel-containing layer, in which the nickel layer, the nickel-containing layer, the gold layer, or the gold-containing layer is on the flat face.
In particular, when the gold layer or the gold-containing layer is on the flat face of the bump, oxidation of the surface of the flat face of the bump can be prevented. Accordingly, this is more preferable since it is not necessary to remove the oxide layer using a dry etching method.
However, when the surface of the electrically conductive material layer composed of the copper layer or the copper-containing layer is directly covered with the gold layer or the gold-containing layer, and when a temperature of 150xc2x0 C. to 200xc2x0 C. is applied in a process for forming the thin-film device after the formation of the bump, copper and gold may diffuse and may mix with each other in some cases.
In the case in which the electrically conductive material layer constructing the bump is the copper layer or the copper-containing layer, and the protective layer covering the electrically conductive material layer is the gold layer or the gold-containing layer formed on the surface of the nickel layer or the nickel-containing layer, the nickel layer or the nickel-containing layer functions as a diffusion-inhibiting layer to prevent copper and gold from diffusing and mixing with each other.
In addition, when the structure is the nickel layer or the nickel-containing layer formed on the surface of the electrically conductive material layer composed of the copper layer or the copper-containing layer, stress can be reduced. Consequently, separation of the electrically conductive material layer and the protective layer hardly occurs.
The electrically conductive material layer constructing the bump may comprise a nickel layer or a nickel-containing layer, and the protective layer may comprise a gold layer or a gold-containing layer.
In addition, the bump may be a single layer structure of the electrically conductive material layer of a nickel layer, a nickel-containing layer, a gold layer, or a gold-containing layer.
The thin-film structure of the present invention has an electrode layer formed on the flat face of the bump and is in electrical contact therewith, and for example, can be used as an electrode for a thin-film device of a thin-film magnetic head.
In the case in which the formation of the oxide layer on the surface of the flat face of the bump can be suppressed, and the oxide layer can be reliably removed from the surface of the flat face of the bump according to the present invention, when the electrode layer electrically contacting the bump is formed on the flat face thereof, or in the case in which the formation of the oxide layer on the surface of the flat face of the bump can be prevented, the cohesion and electrical conduction between the bump and the electrode layer can be improved, and electrical contact defects therebetween can be reduced.
Direct current resistance of a thin-film device having an electrically conductive layer and the thin-film structure of the present invention, such as a thin-film magnetic head, a thin-film inductor, and a thin-film transformer, can be stabilized.
A method for manufacturing a thin-film structure according to the present invention comprises a step (a) of forming a bump on an electrically conductive layer, in which the surface of the bump is composed of at least one of a nickel layer, a nickel-containing layer, a gold layer, and a gold-containing layer; a step (b) of forming an insulating layer on the electrically conductive layer and the bump; and a step (c) of polishing the insulating layer and the bump so as to form a flat face on which one of the nickel layer, the nickel-containing layer, the gold layer, and the gold-containing layer of the bump is at a level of the surface of the insulating layer.
In the present invention, the surface of the bump formed in the step (a) is composed of at least one of the nickel layer, the nickel-containing layer, the gold layer, and the gold-containing layer. Hence, in the step (c), the surface exposed at the surface of the insulating layer of the thin-film device, that is, the flat face of the bump, is composed of one of the nickel layer, the nickel-containing layer, the gold layer, and the gold-containing layer.
In the case in which the flat face of the bump is one of the exposed surfaces of the nickel layer, the nickel-containing layer, the gold layer, and the gold-containing layer, even though the oxide layer forms on the flat face of the bump at room temperature or in heated air, it is believed that the oxide layer will not exceed a certain thickness.
For example, when the flat face of the bump is an exposed surface of the nickel layer or the nickel-containing layer, it has been experimentally confirmed that the thickness of the oxide layer formed on the surface of the flat face of the bump is at most approximately 3.0 xcexcm.
Consequently, in the case in which the nickel layer or the nickel-containing layer is on the flat face in the step (c), the method for manufacturing the thin-film structure further comprises a step (d) of dry etching the surface of the nickel layer or the nickel-containing layer so as to remove the oxide layer thereon. By removing approximately 3.0 nm or more of the oxide layer formed on the flat face of the bump from the surface thereof, the oxide layer is reliably removed from the surface of the flat face of the bump.
Meanwhile, in the case in which the gold layer or the gold-containing layer is on the flat face in the step (c), the oxidation of the flat face of the bump is prevented, and hence, the step of dry etching is not necessary.
In addition, in the step (a), the nickel layer, the nickel-containing layer, the gold layer, or the gold-containing layer is preferably formed on the copper layer or the copper-containing layer so as to form the bump.
When a main body of the bump is primarily formed using copper or the copper-containing material which has superior electrical conductivity due to the low resistance thereof, electrical conductivity of the thin-film structure is superior and high-frequency characteristics of the thin-film device are also improved.
Furthermore, in the step (a), the bump composed of the nickel layer or the nickel-containing layer formed on the surface of the copper layer or the copper-containing layer and the gold layer or the gold-containing layer formed on the nickel layer or the nickel-containing layer is more preferable.
When the gold layer or the gold-containing layer is formed on the surface of the copper layer or the copper containing layer so as to form the bump, copper and gold may diffuse and may mix with each other in some cases.
Hence, it is preferable that the nickel layer or the nickel-containing layer functioning as a diffusion-inhibiting layer be formed between the copper layer or the copper-containing layer and the gold layer or the gold-containing layer.
In the step (a), it is preferable that the nickel layer, the nickel-containing layer, the gold layer, or the gold-containing layer be formed at not less than a predetermined thickness, for example, not less than 5 xcexcm.
When the nickel layer, the nickel-containing layer, the gold layer, or the gold-containing layer is formed to be not less than the predetermined thickness, and when the insulating layer and the bump are polished so that the surface of the bump is flush with that of the insulating layer to form the flat face in the step (c), overpolishing to the point that the nickel layer, the nickel-containing layer, the gold layer, or the gold-containing layer is completely removed from the surface of the flat face is avoided.
The method for manufacturing the thin-film structure of the present invention may further comprise a step, following the steps (c) and (d), of forming the electrode layer electrically contacting the bump on the flat face of the surfaces of the bump and the insulating layer, and as a result, an electrode usable in a thin-film device, such as a thin-film magnetic head, can be formed.
As described in the method for forming the thin-film structure of the present invention, when the oxide layer can be reliably removed from the surface of the flat face of the bump, or when the formation of the oxide layer on the surface of the flat face of the bump can be prevented, the thin-film device can be manufactured having improved cohesion and electrical conduction between the bump and the electrode, and in which electrical contact defects therebetween can be reduced.